This invention relates to thin film transmission dynodes, and in particular to a method of producing such dynodes. The invention also relates to a photomultiplier or imaging device incorporating such a thin film dynode.
In a thin film transmission dynode, secondary electrons are generated by impacting one side of the film with incident electrons. The energy of the incident electrons is adjusted such that the incident electron beam penetrates nearly through the thin film dynode material. This requires high accelerating voltages for the incident electrons and very thin film structures of materials that have small or negative electron affinity. The known thin film transmission dynodes are usually less than 100 nm in thickness and are quite fragile. Consequently, they require special methods for preparation and mounting when used in photoelectronic devices.
FIG. 1 is a schematic diagram of a known thin-film diamond transmission dynode 10. As shown in FIG. 1, a beam of incident electrons 12 is directed toward the incident surface 14 of the thin diamond film 10. The incident electrons 12 traverse the diamond material and produce secondary electrons 16 within the film 10. Some of the secondary electrons 18 are able to diffuse to the opposite surface 19 where they can escape into a vacuum because of the low or negative electron affinity of the diamond surface. However, the process of electron transmission in the known diamond thin film dynode is undesirably inefficient because of scattering losses which limit the diffusion length of the electrons to short distances. The very short electron diffusion lengths mandate that the dynode be limited to not more than about 100 nm thick.
In accordance with one aspect of the invention described herein, there is provided an electron multiplying transmission dynode for a photoelectronic device. The transmission dynode includes a layer of semiconductive material having an input surface and an output surface. A first metallic electrode is formed on the input surface of the semiconductive layer and a second metallic electrode is formed on the output surface of said semiconductive layer. The semiconductive material preferably has a crystalline structure that is textured with a (100) orientation.
In accordance with another aspect of this invention there is provided a photocathode for emitting photoelectrons in response to incident light. The photocathode includes a layer of semiconductive material having an input surface and an output surface. A first metallic electrode is formed on the input surface of the semiconductive layer and a second metallic electrode is formed on the output surface of the semiconductive layer. As in the case of the transmission dynode, the semiconductive material preferably has a crystalline structure that is textured with a (100) orientation.
In accordance with a further aspect of this invention there is provided an optical imaging device. The optical imaging device includes a photocathode, an electron multiplying transmission dynode having input and output surfaces, and a phosphor screen disposed for receiving electrons emitted from the output surface of said electron multiplying transmission dynode. The electron multiplying transmission dynode has a thin layer of a semiconductive material. A first metallic electrode is formed on the input surface and a second metallic electrode is formed on the output surface. The electron multiplying transmission dynode is disposed for receiving electrons from the photocathode at the input surface. The optical imaging device also includes a source of electric potential operatively connected to the first and second metallic electrodes, means for spacing the electron multiplying transmission dynode from the photocathode, and means for spacing the phosphor screen from the output surface.
In accordance with a still further aspect of this invention there is provided a photomultiplier having a photocathode, an electron multiplying transmission dynode, and an anode for receiving electrons emitted from the electron multiplying transmission dynode. The electron multiplying transmission dynode includes a thin layer of a semiconductive material having an input surface and an output surface. A first metallic electrode is formed on the input surface and a second metallic electrode is formed on the output surface. The electron multiplying transmission dynode is disposed for receiving electrons from the photocathode at the input surface. The photomultiplier also includes a source of electric potential operatively connected to the first and second metallic electrodes, means for spacing the electron multiplying transmission dynode from said photocathode, and means for spacing the anode from the electron multiplying transmission dynode.